1. Field of the Invention
The present invention relates generally to an imaging apparatus having an image pick-up device, such as CCD. The present invention relates particularly to an imaging apparatus which is designed to recognize an object so as to allow a taking lens to be kept focused on the object.
2. Description of the Related Art
An imaging apparatus in which focusing of a taking lens focused on an object is maintained even when the object is in motion is disclosed in, for example, U.S. Pat. No. 5,031,049. In that imaging apparatus, the movement of the object is detected on the basis of an electrical signal in a tracking area. The tracking area is moved in response to the movement of the object, and focusing of the taking lens is detected on the basis of part of the electrical signal in that tracking area.
However, in this conventional imaging apparatus, the area to be set for control of the shooting operation is preset on a screen, and thus cannot be set at a desired position by the photographer, thus limiting the shooting range. Therefore, in an apparatus in which an object area (range) is set on the basis of the information on the object, such as focusing information, since the object area is set depending on a focused or non-focused condition thereof, an object which is not intended by the photographer may be selected, i.e., focusing may be conducted on an undesired object.
Automatic focusing devices for video cameras are known, which detect the end definition of a shot screen by the high frequency component of a video signal to control focusing such that the high frequency component is at a maximum. This automatic focusing system is based on the knowledge that the video signal rapidly changes at the edge of an object image, increasing the high frequency component level of the video signal, and that a higher frequency component level corresponds to a sharper object image.
The schematic configuration of such a conventional automatic focusing device is shown in FIG. 33.
In the figure, an object image is formed on a photo image device 502 by a lens 501. The photo image device 502 converts the object image into a video signal. Thereafter, a preamplifier (not shown) amplifies the video signal obtained by the photo image device 502, and outputs it to a signal processing circuit 503. The signal processing circuit 503 processes the signal to obtain a video signal conforming to, for example, NTSC method. A high-pass filter (HPF) 504 picks up the high frequency component from the output of the signal processing circuit 503, and outputs it to a gate setting circuit 505 which selectively passes only the signal of the area in which focusing is detected. A detection circuit 506 detects the output from the gate setting circuit 505 to form a signal indicating the maximum amplitude, i.e., the peak value, in the high frequency component points in the selected area. The output signal of the detection circuit 506 represents the degree of focusing of the lens 501. The higher the signal level, the higher the degree of focusing of the lens 501. A motor driving circuit 507 drives a motor 508 according to the output value of the detection circuit 506 which is produced for each screen to automatically focus the lens 501.
In the thus-arranged automatic focusing device, if both a distant object and a nearby object are present in the focus detection area, a focusing failure caused by a combination of distant and nearby objects may occur, making the lens focused on an undesired object or unstably focused. In order to eliminate such defects, Japanese Patent Application Laid-Open No. 3-154576 discloses the automatic focusing device which includes means for recognizing the shape of an object on which the lens is focused and in which the focus detection area is determined on the basis of the result of the recognition.
The above-described focusing failure caused by a combination of distant and nearby objects may be avoided by setting a focus detection area which is slightly larger than the recognized shape of the object. However, if changes in the shape or movement of the object are large or if the amount of camera shake is large, the focus detection area may deviate from the object, causing focusing failure caused by a combination of distant and nearby objects and hence unstable focusing, as in the case of the conventional device. In addition, where it takes time for the object shape recognition means to recognize the shape of the object, a time lag is increased, thus causing deviation of the focus detection area from the object.
In the above-described conventional automatic focusing device, if the photo taking lens is a zoom lens, a change in the size or position of the object on the screen by zooming impairs the focusing accuracy.
In a practical operation, when zooming is performed from the telephoto end to the wide-angle end, the size of the object on the screen decreases, allowing another object or background to enter the focus detection area. In that case, if such an object or background has a high contrast, it makes focusing unstable, thus reducing the focusing accuracy. Conversely, when zooming is conducted from the wide-angle end to the telephoto end, the object deviates from the focus detection area, making the luminance of the object have less contrast. This also makes automatic focusing unstable.